How to remove errors on a web page. How to fix a web page error

Using F12 Developer Tools to Debug JavaScript Errors

This content links to more old version F12 developer tools. Download the latest F12 Tools documentation.

F12 tools allow developers to quickly debug JavaScript code without leaving the browser. Built into every instance Windows Internet Explorer 9 F12 Developer Tools provide debugging tools such as breakpoints, viewing control and local variables, as well as a console for messages and immediate code execution.

This article discusses how to use F12 developer tools to debug JavaScript code. This article is not exhaustive teaching aid on debugging, its goal is to highlight tools that can help a developer get started working with the generated code. IN Internet Explorer 9 Press F12 to open the developer tools and click the Script tab to get started.

The Script tab on the left contains a source code area where a developer can view their JavaScript code, set breakpoints, and step through their functions. In the right pane, you can switch between tabs for Console, Watch Variables, Local Variables, Watch Stack, and Breakpoints.

Starting and stopping the debugger

When you first open F12 Developer Tools and click the Script tab, your code appears on the left side and the console on the right. You may see a message in the console that says, "Please refresh the page to view messages received before you opened the F12 Developer Tool." When you refresh the web page, the console will display any errors or warnings caused by the browser.

To set breakpoints, view control and local variables, and see the call stack of a sequence of functions, click the Start Debugging button. Clicking the Start Debugging button refreshes the web page and reruns the code in the debugger.

Using the console to detect syntax and other code errors

In most programming projects, errors are divided into syntax, logical, and data entry errors. The console view shows JavaScript errors and exceptions, as well as DOM exceptions. Within your code, you can use the console object to report program status and error messages to the console instead of "alert()" calls or available screen space. For example, you could add the line

Window.console.log("The file opened successfully");

In your JavaScript code to get information about the state of the script without interrupting its execution. Additional information: .

Correction appearance scenarios

F12 Developer Tools can debug JavaScript at the line or statement level, regardless of how the code is displayed. You can even step through scripts that are compressed so they look like large blocks of code. But sometimes it's hard to follow the logic when the code is one block.

To format the script, click the Configuration button and select Format JavaScript. The following screenshots show a block of JavaScript code before and after formatting.

Interrupting code execution

Setting breakpoints in the F12 developer tools is similar to setting breakpoints in binary code debuggers such as Microsoft Visual Studio. In the left pane, click to the left of the line of code where you want to stop. Breakpoints are toggleable, so clicking adds a breakpoint and clicking again removes it.

You can add as many breakpoints to your code as you need. You can either right-click a line of code and select Insert Breakpoint, or click in the left margin next to the statement where you want to set a breakpoint.

Using the F12 authoring tools, you can set a breakpoint at the statement level, even if those statements are in a block or line of multiple statements. This allows you to create a breakpoint in a compressed code segment. The best way to set a breakpoint under these conditions is to right-click the code and select context menu item Insert breakpoint. You can also use the script formatting (pretty typing) described earlier, formatting the lines to make them easier to click in the margins.

Manage multiple breakpoints using the Breakpoints tab

If you have a large amount of code with many breakpoints, or even multiple files, you can use the Breakpoints tab to keep track of all the breakpoints you need. On the Script tab, click the Breakpoints tab in the property or right pane. See the following image for an example:

The Breakpoints tab allows the developer to enable or disable, delete, select, and copy breakpoints without having to navigate to the exact locations of the breakpoints. To enable or disable a breakpoint, select or clear the check box next to the option you want to change. You can also jump directly to a breakpoint in your code by double-clicking it in the list. You can select multiple breakpoints at once by pressing the CTRL key and clicking the breakpoints you want.

The Breakpoints tab also provides a context menu (opened by right-clicking) that allows you to delete, enable, disable, or copy groups of breakpoints. These parameters are shown in the following table.

Menu item Action

Delete	Deleting a breakpoint permanently.
Delete everything	Removes all breakpoints permanently.
Enable All	Select all checkboxes in the list.
Disable all	Clears all checkboxes in the list.
Condition	Allows you to set a conditional breakpoint for a single point. This option is not available if multiple breakpoints are selected.
Copy	Copies the text of the descriptions of the selected breakpoints.
Select all	Highlights all breakpoints in the list.
To source code	Moves to the left code area to display the selected breakpoint. This option is not available if multiple breakpoints are selected.

Conditional breakpoints

Stopping unconditionally on a line of code is useful, but stopping when a property or variable reaches a specific value is even more powerful. To stop when a specific value is reached or set, create a breakpoint and then open the Breakpoints tab. Right-click the breakpoint you want and select Condition.

In the conditions dialog, add the correct JavaScript operator. The executing code will stop at this breakpoint when the statement evaluates to true. For example, in the following image, code execution stops when the value of the oAudio.paused property becomes false.

You can specify a single condition or using logical operators, create a more complex stopping condition. But remember that the scope of variables and objects will remain the same as when they were displayed in the breakpoint viewport. A condition that is not in scope will not evaluate to true.

Stepping through the code

When code execution stops at a breakpoint, you can use the jump buttons to continue execution (F5), interrupt execution (CTRL+SHIFT+B), step into a function (F11), skip a function (F10), or exit a function (SHIFT+F11 ). When execution is stopped at a breakpoint or during step-by-step execution, the debug window effectively becomes modal.

Therefore, before continuing to interact with the web page, you need to stop debugging (SHIFT+F5) or continue execution (F5). This is something to keep in mind if a web page seems to have stopped responding. If, when multiple windows are open, the debugger window is not on top, it may be waiting for a response at the breakpoint. In this case, to take back control of the web page, locate the debug window for that web page and press F5 to continue or press SHIFT+F5 to stop debugging.

View variables using the Watch and Local tabs

The Watch tab allows you to define and view watch variables in the code you are debugging. It contains the name, value and type of the specified variables. You can click the line labeled Click to add... on the Watches tab and enter a variable name. If you don't want to type the variable name, you can copy and paste it into the list of control variables.

The values ​​in the list of watch variables are displayed regardless of whether the code is currently being debugged or not. When you enable debugging and code tracing or set breakpoints, the scope of the variable values ​​in the list is determined by the current point in the script's execution. If debugging is disabled, the scope is global and only global variable values ​​will be shown.

Unlike the Watch values ​​tab, which shows values ​​regardless of whether the corresponding variables are in scope, the Local tab shows only variables in the current scope. You don't need to add any variables to view because when you change the scope, this tab updates to show all available variables.

To see the difference, open the following example in Internet Explorer 9 and follow these steps.

JavaScript debugging example //create a global variable for our var display; function init() ( //initialize only after the HTML has been loaded display = document.getElementById("results"); ) function firstParam() ( //set breakpoint here var a = 5; secondParam(a); ) function secondParam (a) ( var b = 10; thirdParam(a, b); ) function thirdParam(a, b) ( var c = 15; var d = a + b + c; //display to console if F12 tools is open if (window.console && window.console.log) ( window.console.log(a + " + " + b + " + " + c + " = " + d); ) else ( display.innerText = a + " + " + b + " + " + c + " = " + d; ) )

Run

Download the example in Internet Explorer 9.

Press the F12 key to open the F12 Developer Tools and click the Script tab.

In the left pane, scroll to the first function, right-click on the line "var a = 5;" and select Insert Breakpoint.

Click the Start Debugging button, and then on the web page in your browser, click the Run button.

In the F12 developer tools, click the Watches tab on the right side and add the variables "a, b, c, and d.".

Step through the code by pressing F11 or the Step In button, and view the variables in the Watches tab.

As you step through each function, the values ​​being viewed should change from undefined to some values.

To get a feel for the differences from the Local tab, press F5 to continue from F12 Developer Tools. On the web page in your browser, click the Run button to run the code again and return to the F12 Developer Tools. In the right pane of the Script tab, click the Local tab and press F11 to step through the functions again. Note that the list of local variables only contains variables that have values. The Local view also shows the arguments passed to the function, their value, and their type.

View the call stack

The Call Stack tab allows you to view the path taken when calling functions from code. This can help detect unexpected code paths resulting from a bug. On the Call Stack tab, you can double-click any function and navigate to its call in the source code.

Try the above example and keep an eye on the Call Stack tab as you step through the code into functions.

In the Call Stack tab, the current function or location is always on top (it is indicated by an arrow in both the Call Stack tab and in the code fields). When you double-click any of the functions in the list, the operator that calls the function is highlighted.

Debugging multiple scripts

Large web pages typically use multiple JavaScript files. The F12 Developer Tools allow you to work with multiple script files, just as you would when debugging your code. To view another file, click the down arrow next to the Start Debugging button to display a list of scripts associated with this web page. When using the F12 developer tools, code will be stepped through all files. You can add variables from any script file to view, and the call stack view will show the execution path through the functions contained in the various script files.

Changing the Document Mode setting

The Document Mode setting on the right side of the Menu bar is available on every F12 Developer Tools tab, but this setting is especially useful when debugging code in the Scripts tab. Internet Explorer 9 lets you change document mode to emulate previous standards Windows versions Internet Explorer. In Internet Explorer 9, exiting the declaration sets the document type to compatibility mode by default. When working with new opportunity or a new standard such as HTML5, some errors may appear to be programming errors but are actually caused by a missing or incorrect doctype declaration.

The F12 Developer Tools for Internet Explorer 9 can't fix your code for you, but they do make finding JavaScript errors a little easier.

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Note: This article was updated on 2019-12-22 and was previously published under WIKI_Q210794

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Quite often, users may experience a situation where a script error message appears in the Internet Explorer (IE) browser. If the situation is isolated, then there is no need to worry, but when such errors become regular, then it is worth thinking about the nature of the problem.

A script error in Internet Explorer is usually caused by the browser incorrectly processing the HTML page code, the presence of temporary Internet files, account settings, and many other reasons, which will be discussed in this material. Methods for solving this problem will also be discussed.

Before you begin with the common methods for diagnosing Internet Explorer problems that cause script errors, you need to make sure that the error is not just occurring on one specific site, but on multiple web pages at once. You also need to check the web page on which the problem occurred. this problem under another account, on a different browser and on a different computer. This will allow you to narrow down the search for the cause of the error and exclude or confirm the hypothesis that the messages appear as a result of the presence of certain files or settings on the PC

Blocking Internet Explorer active scripts, ActiveX and Java

Active scripts, ActiveX and Java controls affect the way information is generated and displayed on the site and can be the real cause of the previously described problem if they are blocked on the user's PC. To ensure that script errors are occurring for this reason, you simply need to reset your browser's security settings. To do this, follow the following recommendations.

• Open Internet Explorer 11
• Service

• In the window, go to the Security tab
• Next, click the Default button, and then the OK button

Internet Explorer temporary files

Every time you open a web page, Internet browser Explorer saves a local copy of this Internet page on your PC in so-called temporary files. When there are too many such files and the size of the folder containing them reaches several gigabytes, problems with displaying the web page may occur, namely, a script error message may appear. Regularly cleaning out your temporary files folder may help resolve this issue.
To delete temporary Internet files, follow these steps.

• Open Internet Explorer 11
• In the upper corner of the browser (right), click the Tools icon in the form of a gear (or the key combination Alt+X). Then in the menu that opens, select
• In the window, go to the General tab
• In the Browsing History section, click the Delete... button.

• In the Delete browsing history window, check the boxes next to Temporary Internet files and websites. Cookies and website data, Magazine
• Click the Delete button

Antivirus software operation

Script errors are possible through operation antivirus program when it blocks active scripts, ActiveX and Java controls on a page, or temporary browser file folders. In this case, you need to refer to the documentation for the installed anti-virus product and disable scanning of folders to save temporary Internet files, as well as blocking of interactive objects.

Incorrect processing of HTML page code

It usually appears on one specific site and indicates that the page code is not completely adapted to work with Internet Explorer. In this case, it is best to disable script debugging in the browser. To do this, follow these steps:

• Open Internet Explorer 11
• In the upper corner of the browser (right), click the Tools icon in the form of a gear (or the key combination Alt+X). Then in the menu that opens, select
• In the window, go to the Advanced tab
• Next, uncheck Show a notification for every script error and click OK.

This is a list of the most common reasons, which cause script errors in Internet Explorer, so if you are tired of such messages, pay a little attention and solve the problem once and for all.

There are, of course, others - I will definitely mention them.

Firebug for Firefox

I don't know for sure whether FireBug is the ancestor of other tools for developers, but it is definitely the most popular, convenient and functional in .

Firebug is an add-on for Firefox, which means you need to download it from the Firefox add-ons website and install it.

To trigger a firebug, just press F12.

Features of this add-on:

• Inspecting and editing dynamically changing HTML;
• On-the-fly CSS editing;
• Debugging JavaScript, command line to execute scripts;
• Monitoring network requests - you can see the sizes and download times of files and scripts, request headers;
• DOM parser.

You can talk for a long time about these possibilities in detail, but I think that all our readers know them, and if not, detailed information is on Firebug’s home page or the same thing in Ilya Kantor’s translation.

In addition to the firebug itself, you may need a useful add-on to it - FireCookie, with which (surprise :-) you can view and change cookies.

WEB Developer Toolbar for Firefox

Another useful addition to Ognelis. It looks like this:

Let's look at it point by point.

Disable

Allows you to turn off JavaScript and disable the use of the cache, which is very useful during development (allows you to be sure that the page loaded along with latest updates), cancel the colors used on the page and replace them with standard ones, prohibit sending in the referrer header (the page from which the referral was made).

Cookies

A useful option for working with cookies: you can view, delete, deny and add them.

CSS

This menu stores the coolest feature of the Developer Toolbar - editing CSS on the fly. In addition, it is possible to view css, disable, and so on and so forth. In my opinion, it is very useful to have shortcut keys(CTRL+SHIFT+C, for example, allows you to go directly to the page styles view)

Forms

Everything for working with forms: show passwords, show information about forms, convert form methods (GET » POST and vice versa) and much more. A useful function is “Populate Form Fields” for automatically filling out form fields (for example, when testing a site, when the function of remembering passwords is disabled. Otherwise, I don’t see anything useful in this item.

Images

There is a useful feature to disable images - to see what your site looks like without images. You can circle pictures, show their sizes, and show alt attributes.

Information

There are a lot of options in this menu. The function of displaying the class and id attributes on the page may be useful. In addition, the “View Color Information” item is interesting - to quickly obtain information about the colors that are used on the page. “View document size” - view the page size. “View Response Headers” - view page headers.

Miscellaneous

The most frequently used function is clearing the cache. In addition, the functions available here are “Page ruler” - a ruler, “Page Magnifier” - a magnifying glass and “Line guides” - several lines that can be useful for trimming a template.

Outline

Highlighting different page elements - tables, headings, links, frames, blocks. Resize allows you to resize the browser window to fit any standard screen extensions. Tools features for page validation are stored here. Both local and external. Convenient and quick access to validation of HTML, CSS, etc. To validate HTML, you can use the keyboard shortcut CTRL+SHIFT+H.

View Source

View source code. Possibility of viewing in an external application, viewing the generated code.

I like the one in the right corner the most. It is a fast HTML, CSS validator and JavaScript error indicator. If there are no problems, the icon is green, and if there are problems, the icon is red.

Internet Explorer Developer Toolbar

Starting from 8.0, error debug is already built into this browser. It is called easily by pressing the F12 key. True, it’s as poor as a program from the 90s.

But there is a much cooler tool for this browser, the so-called Internet Explorer Developer Toolbar, which can be downloaded from the link.

In appearance, this toolbar, of course, looks like firebug, but, alas, it has not yet matured to it. Although, on the other hand, it has some capabilities that firebug does not have. I would call Internet Explorer Developer Toolbar a kind of hybrid of Firebug and FireFox WEB Developer Toolbar.

As in firebug, it is possible to inspect an element with a simple click. But, if in we can immediately see padding and margins, here there is no such opportunity.

Additionally, Internet Explorer Developer Toolbar does not dynamically update the element tree like Firebug does. That is, if we change anything on the page using js, we won’t see anything using this toolbar.

What you can enjoy is changing CSS on the fly (an easy way to find what to hack :), the ability to disable CSS and images, the ability to quickly clear the cache and play with cookies, quick access to validation.

The best part: there is a built-in color picker, which allows you to get any color from the page using a pipette. (there is a separate ColorZilla plugin for ff).

Debug DebugBar for Internet Explorer

DebugBar for Internet Explorer can be downloaded from the given link.

An interesting extension in its own right. Installed as an additional panel to the browser:

For some reason there is a built-in search engine, an eyedropper, the ability to change the size of the window and, again, for some reason the ability to send a page to a friend for soap. Although this may be useful. But I failed to take advantage of this opportunity.

In addition, there is an inspector:

The developers were not satisfied with the method of inspection by clicking or pointing: they came up with a more interesting thing. In DebugBar you need to drag the crosshair onto the desired element to see it in the tree. There is no ability to edit CSS. But there is a validator and a built-in js console.

And if you dig into the settings you can find this:

Both funny and sad.

It is known that the Developer Toolbar will be built into the eighth explorer. It will be similar to the one described in the third paragraph, but we hope that it will be better.

Debug DragonFly for Opera

DragonFly is built into Opera starting from version 9.5, so there is no need to install it. In order to activate Dragonfly, go to Tools → Advanced → Developer Tools. And if in English, then Tools → Advanced → Developer Tools.

I’ll warn you right away that DragonFly is in the Alpha2 stage, this explains many of its glitches.

Features list:

• DOM inspector;
• Inspect by click (again, we won't see indentation like in FireFox);
• Editing ;
• Quick access to the error console.

DF is something like a separate page in a frame. If you open it, it will be open for all tabs (unlike firebug). Therefore, before inspecting an element, we must select from the list the page that we want to view.

Unfortunately, here, as in Internet Explorer Dav Toolbar, dynamically created elements are not displayed. And in general, when we inspect the page, no JavaScript is launched: links and buttons are not clicked. Let's hope we see all of these features when DragonFly gets closer to release.

Debug WEB Inspector in Safari

I’ll say right away that about Safari browser I missed the information, so I am not responsible for the adequacy of the material, as they say.

In order to enable the “Development” item in the Safari menu, you need to enable the corresponding item in the settings (the “Advanced” tab):

In the “Development” menu the following functions are available to us:

Let's take a closer look at the WEB inspector:

By default, the inspector opens in HTML view mode. But it can be switched to DOM view mode. For this purpose, there is a switch on the top plate. When you hover over an element in the inspector, it will be highlighted on the page itself. You can't see padding, change markup or CSS on the fly, or see dynamic changes in DOMe on the fly like you can in FireBug. But, you must admit, it looks very cute.

If you want to work with the inspector in a browser window, you can click on the button in the lower left corner.

Even in Safari, a function such as “Network Timeline” is available (the “Network” button in the inspector):

You can clearly see when and how long it takes to download files. You can also view request headers, but, unfortunately, you cannot view the content itself.

Debug for developers in Google Chrome

Lame was born in an advanced form, and it immediately has, albeit crooked for now, but still tools for developers.

• DOM Inspector;
• javascript debugger;
• JavaScript Console.

In order to inspect any element, you need to right-click on it and select “View element code” in the context menu:

The functionality is the same as in Safari: elements are highlighted when hovered, but CSS and HTML editing is not available, and changes in the DOM are not tracked. But the button in the lower left corner, which should attach the inspector to the browser window, does not work.

In the “Resources” tab we can see the following:

Slightly different from the scale in safari. Translucent in this diagram shows the relative file sizes, and full color shows the loading time. One way or another, it is obvious that this part of Chrome is far from finished.

In this article, I looked at the most famous browser extensions and built-ins.

There are others, for example:

• Internet Explorer WEB Development Helper - a good assistant for ASP.NET developers (Internet Explorer);
• WEB Developer Toolbar - toolbar for Internet Explorer and FireFox. There are several useful functions;
• WEB Accessibility Toolbar - toolbar for Internet Explorer. Nothing interesting.

If there are add-ons that I didn’t mention, but they should, or there are functions for the mentioned extensions that I missed, please write.

Enjoy it for your health!

At the moment, there are a large number of different browsers that, to one degree or another, support existing HTML standards. Personally, I prefer the browser from Mozilla Corporation. This browser has a long history (it is based on the famous Netscape Navigator browser). I also like this browser because it supports a system of plugins - separately distributed add-ons, when connected, you can change the functionality and customize it to suit your needs. And finally, this browser is freely distributed and open source, which is also important. That's why I decided to describe the possibilities that this browser provides not only the user, but also the developer of Web pages, how easy and convenient the process of debugging written products can be.

Mozilla Firefox is one of the most popular browsers among developers and web-developers. It attracts their attention because of the opportunities that this browser provides for debugging created projects, fixing errors, and improving them. The standard browser package includes a java console (or “error console”). This utility allows you to debug embedded java-scripts. But, much greater functionality is provided to the browser by third-party plugins, which can be downloaded and installed from the official website of the Mozilla Foundation. Now I want to look at two of these plugins - Web Developer and Firebug. Both of these addons can be downloaded from the above links from the official addons website. After installing them and restarting the browser, the developer has a wide range of possibilities, which I will describe below, in turn for each of the plugins.

Firebug plugin

As stated on the official page: “Firebug integrates with the Firefox browser to greatly enrich the developer toolkit. You'll be able to edit, debug, and examine CSS, HTML, and Javascript live on any web page." And this is true. Let's look at some of the functions of this plugin, namely:

• Viewing and editing HTML.
• Building CSS.
• Network Request Monitoring
• Debugging JavaScript
• JavaScript Research
• Logging for JavaScript

This is not a complete list of all its capabilities. Since the project is open source, anyone can change and add functionality.

To work with the plugin, you must press the F12 key (Ctrl-F12 to work with it in a separate window). After a successful launch, we get the following - Figure 1a, 1b.

Figure 1a. The initial window of the firebug plugin.

Figure 1b. The initial window of the firebug plugin.

Next, the actual work with the plugin begins. Let's say we need to find this or that object in the HTML code, or determine how exactly it is implemented with using CSS current fragent. To do this, you simply need to select the necessary options in the plugin window menu with the mouse. The example below is HTML in Inspect mode. Now, going to the document page, under the mouse cursor we will notice a rectangular area, illustrating the area with which we are working. In the plugin window we will see the HTML and CSS parameters that are used. Also, by clicking on each of them, you can make changes and track them over time. The described actions are illustrated in Figures 2,3,4.

During the development of this project, the plugin described above was used precisely for these purposes. However, although no java script was used here, the firebug plugin can be used to debug it as well. An example of debugging is shown in Figure 5.

As described above, the functionality of this extension is not limited to this. You can study it in full and use it for your needs by downloading it from the links provided, after installing Mozilla Firefox.

Web Developer plugin Web Developer is the second extension for Mozilla browser Firefox is very powerful and functional, allowing for fast and efficient debugging. After installing it, an additional toolbar will appear in the browser window, shown in Figure 6.

Further work with the plugin is intuitive. For example, if we need to do some CSS work (though not as fully functional as the Firebug plugin), we can click CSS menu and block, enable or disable.

It is very convenient for the developer to be able to see how his project will look on monitors with different resolutions. To do this you need to use the Resize tab. Here you can manually set the required screen resolutions (800x600, 1024x768, etc.), and then freely switch between them, zoom in or out of content. This functionality is depicted in Figure 7.

This extension also has rich functionality, which are all described and available on official websites.

List of sources used

• 1. www.getfirebug.com
  Official website of the add-on.
• 2. http://addons.mozilla.org
  The official website of Mozilla, where plugins for Firefox browser, information for developers, information on using plugins.
• 3. http://chrispederick.com/work/web-developer/
  Official website of the plugin developer WebDeveloper.